The Supreme Court’s hottest environmental case of the year pits Texas against California on Monday, and that’s just for starters.

More than half of the nation’s states have taken sides in a dispute over federal authority to regulate stationary greenhouse gas emissions. Conservative lawmakers such as Senate Minority Leader Mitch McConnell, R-Ky., hold one position. Southern California air pollution managers defend another.

“It’s the big one,” said Washington, D.C.-based attorney Peter S. Glaser, an energy law expert, adding that greenhouse gas regulation “seems to have become this central, major, international issue.”

Underscoring the potential stakes, the unusually long 90-minute oral argument, set for Monday morning, consolidates six lawsuits that challenge Environmental Protection Agency rules. The lawsuits have been boiled down to one central question: Did the EPA overstep its bounds in regulating stationary greenhouse gas emissions based on an earlier determination that it could regulate such emissions from motor vehicles?

Texas Solicitor General Jonathan F. Mitchell says the EPA went too far. He’ll be one of two attorneys Monday attacking the environmental agency’s regulations.

“EPA believes it can disregard unambiguous, agency-constraining statutory rules and unilaterally establish a new regulatory regime to deal with novel environmental challenges,” Mitchell declared in one key legal brief, filed along with Texas Attorney General Greg Abbott.

The Obama administration is defending the EPA, supported by a number of states and environmental organizations.

“EPA has the authority to determine how best to protect public health and welfare while devising mechanisms to ease the regulatory burden on the states,” according to a legal brief filed on behalf of California, New York, Washington and 12 other states.

The case is complicated and the outcome far from clear-cut, though Glaser stressed that even if industry wins, “I don’t see the decision cutting off greenhouse gas regulations completely.” Still, the case is the crucial next step in clarifying an important Supreme Court environmental decision from 2007.

In that earlier case, a closely divided court held that the Clean Air Act gave the EPA authority to regulate greenhouse gases, which contribute to global climate change. A hotter planet, in turn, has been linked to worsening ozone pollution, more intense forest fires, increased drought, extreme storms and a host of human respiratory problems, among other things.

“The EPA determined that greenhouse-gas emissions endanger public health and welfare in ways that may prove to be more widespread, longer-lasting and graver than the effects of any other pollutant” regulated under the Clean Air Act, Solicitor General Donald Verrilli Jr. noted on behalf of the Obama administration.

Targeting six greenhouse gases, including carbon dioxide and methane, the EPA set tailpipe emission standards for cars and light trucks.

Here it gets a little tricky.

The EPA further reasons that since the six greenhouse gases have been deemed dangerous enough to regulate as tailpipe emissions, they must also fall under the pre-construction permit requirements for stationary emission sources, such as small industrial plants and agricultural facilities.

The new requirements for stationary emission sources started in 2011.

“This is a procedural and a bureaucratic nightmare, with no benefits in sight,” Washington, D.C.-based attorney C. Boyden Gray said Thursday.

Gray, a former White House counsel for President George H.W. Bush, authored a brief opposing the EPA regulations on behalf of Kansas and five other states.

Texas has asked the Supreme Court to consider overturning its 5-4 decision in the 2007 case, an aggressive step that the precedent-conscious court probably will be loath to consider. Rather, the court seems likely to look most seriously at how the EPA expanded on the tailpipe emission regulations.

The Clean Air Act sets 100 or 250 tons per year, depending on the source, as the pollutant emissions threshold for when permits are needed. For greenhouse gas emissions, which are emitted from many sources, the EPA changed this to a more lenient 100,000 tons per year.

Keeping the stricter emission permit standards would have meant that tens, if not hundreds, of thousands of additional sources would face more regulatory burdens, according to the EPA.

“The power asserted by the EPA here is nothing less than a unilateral power to change the text of duly enacted statutes,” attorney Charles J. Cooper wrote in a brief filed for McConnell and other Republicans.